In recent years, recording video data into recording media such as CDs and DVDs and reading and reproducing the recorded data if needed have been widely practiced.
As an amount of video data is enormous, data compression by encoding is generally performed in the case of recording the data on the recording media.
As an encoding method for the video data, there is inter-frame prediction coding to generate a predicted image by using a motion vector detected from an encoding target image and an image temporally before or after this encoding target image, and to encode a difference between the encoding target image and this predicted image in accordance with the MPEG standard.
In order to reduce an amount of calculation for motion vector detection, Japanese Patent Laid-Open No. H09-65340 discloses a technique in which motion vectors having small distortion are selected from multiple motion vectors detected with low pixel accuracy and then motion vectors with high pixel accuracy are detected based on the selected motion vectors, for example.
By utilizing this technique, in the process of inter-frame prediction coding, it is possible to reduce the number of motion vectors to be detected with high pixel accuracy from 8 to 4 in the case of generating a B picture or to reduce from 4 to 2 in the case of generating a P picture.
As a consequence, it is possible to reduce the amount of image data to be outputted for performing detection with high pixel accuracy as well as the amount of calculation for detecting second motion vectors, thereby reducing a load on a motion vector detection device.
However, when the second motion vectors in the inter-frame prediction coding are detected by using the technique disclosed in Japanese Patent Laid-Open No. H09-65340 described above, a less number of fields are referenced. This may bring about a problem of degrading the image quality.
In Particular, when the P picture is generated as the predicted image, there is a problem of substantial degradation of the image quality because the number of frames to be referenced in the second motion vector detection is 2.